1. Field of the Invention
The present invention relates, in a general aspect thereof, to a method of building a belt structure in a tyre, in particular for motorcycle wheels.
2. Description of the Related Art
Tyre building for vehicle wheels involves formation of a carcass structure essentially made up of one or more carcass plies substantially shaped in a toroidal configuration and having their axially opposite side edges in engagement with respective annular reinforcing structures incorporating circumferentially inextensible annular elements, usually referred to as “rings”.
Applied to the carcass structure, at a radially external position thereof, is a belt structure comprising one or more belt strips in the form of a closed ring, essentially made up of textile or metallic cords which are suitably oriented with respect to each other and to the cords belonging to the adjacent carcass plies.
In addition, a tread band is applied to the belt structure at a radially external position thereof, which tread band is usually made up of a strip of elastomer material of appropriate thickness.
To the aims of the present specification it should be pointed out that by the term “elastomer material” it is intended a rubber blend in its entirety, that is the assembly made up of at least one base polymer suitably amalgamated with reinforcing fillers and/or process additives of various types.
Finally, a pair of sidewalls is applied to the opposite sides of the tyre being formed, each of said sidewalls covering a side portion of the tyre included between a so-called shoulder region, located close to the corresponding side edge of the tread band, and a so-called bead located at the corresponding annular reinforcing structure.
While tyres for cars or trucks are characterised by a substantially flattened cross-section outline at the tread band, tyres for motorcycles are distinguishable due to their marked transverse curvature or bending, usually defined by the particular ratio value between the distance of the tread centre from the line passing through the opposite side extremities of the tread itself, measured at the equatorial plane of the tyre, and the distance measured along the tyre chord between said extremities. In tyres for two-wheeled vehicles the value of the bending ratio generally is at least as high as about 0.15 and it is usually in the order of 0.3 in the case of rear tyres, and even higher, until 0.45 in the case of front tyres, against a value usually smaller than 0.05 in tyres for motor-vehicles.
Formation of the belt structure is usually accomplished on an auxiliary metal drum, a “comb drum” for example or a smooth drum, on which one or more belt strips are formed and radially superposed to define a so-called belt ring, to which the tread band is then applied. In building of tyres having a high transverse curvature, formation of at least one belt strip is provided which is obtained by circumferentially winding up at least one textile or metallic and optionally rubberized cord into a plurality of coils disposed in axial side by side relationship according to a winding angle of substantially zero value with respect to the equatorial plane, on expandable sectors of the auxiliary drum already in a radially expanded position and conveniently shaped so as to correspond to the curvature outline of the tyre.
After carrying out application of the tread band, the belt ring-tread band assembly is picked up from the comb drum to be coaxially fitted on the carcass structure previously made in the form of a cylindrical sleeve on a building drum. The carcass structure is then submitted to a conformation step by radial expansion of the carcass plies in a toroidal outline, concurrently with axial approaching of the annular reinforcing structures. During this step, the carcass plies adhere to the inner surfaces of the belt ring previously disposed in coaxial relationship around the carcass sleeve.
In the case of a belt structure having cords oriented with a substantially zero angle relative to the equatorial plane of the tyre being formed, it is not possible to lay down the cords on a cylindrical surface and thus give said surface a toroidal conformation, because said circumferential orientation of the cords does not enable elongation of the latter to the extent required by conformation, in motorcycle tyres. Therefore the cord must be formed into spirals directly on the outline of the carcass or the drum having already reached a toroidal conformation.
Up to now it has been found it convenient, for process reasons, not to wind up one single cord, but to simultaneously wind up several cords in the form of a strap. Exactly straps are ribbons of elastomer material incorporating two to ten cords at least partly. Actually, laying down of bare cords with a substantially zero angle relative to the equatorial plane of the tyre would lead to slipping of same out of the desired position during the winding step on the toroidal outline, exactly due to their precarious position.
Therefore forming the cords into a strap has the function of ensuring maintenance of the desired position by the cords during all the tyre preparation steps.
For the above aim in document EP-A-0 718 122 in the name of the same Applicant, application of a thin layer of elastomer material (laying layer) filled with aramid fibres or the like is suggested before winding up of the coils. In more detail, this layer is formed by winding up on the auxiliary drum, a thin blend sheet obtained from calendering, of a thickness included by way of example between 0.075 and 0.5 mm, and of a width substantially corresponding to the width of the belt structure to be obtained.
The Applicant has found it useful to utilise, in making the belt layer as above described, cords of the so-called high starting elongation type, for example metallic cords of the HE or HI type, as commonly identified in the field, or elastic textile cords.
The abbreviation HE means “High Elongation” and points out that winding of the strands in the cord and the wires in each strand is carried out in the same direction (according to the so-called Lang's Lay construction).
In addition, winding is of such a nature that small movements of the strands relative to each other are allowed. In this way the finished cord substantially acquires a particular load-elongation curve of its own which has an important change of slope, usually called “knee”, in the elongation values on achieving a given load. In particular, the starting line of the curve shows very important elongations for small load variations, and is connected through the knee to the final line of the curve showing very small elongations for important load variations.
A cord of this type is described in document EP-B-461 646, in the name of the same Applicant. The centre line value of the knee conditions the correct building and moulding of the tyre and, to this aim, it usually corresponds to an elongation included between 0.4% and 1%.
The abbreviation HI identifies cords in which at least one of the wires is deformed due to bending along its longitudinal extension, so as to create gaps where rubber penetrates into the cord. This type of cord has an intermediate behaviour (defined by the curve load/elongation) between that of the cords in opposite directions (of the Regular Lay type) and that of the cords in the same direction (of the Lang's Lay type).
As an alternative to the metallic cords, use of elastic textile cords is provided, made of Kevlar (polyethylene terephthalate) for example. These cords are made up of bundles of filaments of organic material, suitably twisted so as to achieve behavioural features similar to those of the above described metallic cords.